In a contactless power supplying system that performs electromagnetic induction method, an electric appliance including a power receiving device is set on a setting surface of a contactless power supplying device. In this state, the contactless power supplying device excites its primary coil thereby exciting a secondary coil arranged in the power receiving device of the electric appliance through electromagnetic induction. Secondary power is generated by the secondary coil and converted into direct current power in the power receiving device. The DC power is supplied to a load of the electric appliance as driving power.
The contactless power supplying device may include a metal detector that detects metal foreign objects and stops supplying power when the metal detector detects a metal foreign object. This prevents heating of a metal foreign object. When a metal foreign object is arranged between the contactless power supplying device and the electric appliance (power receiving device), the metal foreign object may be inductively heated when power is being supplied.
Japanese Laid-Open Patent Publication No. 2000-295796 describes an example of a metal detector arranged in a contactless power supplying device. The metal detector described in the publication excites the primary coil of the contactless power supplying device at a predetermined frequency (210 kHz), which differs from an excitation frequency (180 kHz) when supplied with power. The metal detector detects the presence of a metal foreign object when a change occurs in the inductance of the primary coil excited at the predetermined frequency and stops supplying power when a metal foreign object is present.